The ADCOM GTP-500 II is an analog preamplifier with a built-in AM-FM stereo tuner. Use it as your control center on your desk, add a remote power amp and speakers, and you've got a first-rate hi-fi system.

A pleasant feature of this all-analog preamp is a remote control. The tuner is a quartz synthesizer, the volume control is a motorized potentiometer, and input switching is electronic. Put this in your rack, retire to your chair with the remote, and enjoy it in your music room.

Plug your power amps and the rest of your system into the switched outlets on the back, and with the optional remote control you can turn everything on and off from across the room.

ADCOM pitched these in the 1990s as "the cure for the common receiver," because when teamed up with any power amp, like ADCOM's own, you have a complete AM-FM stereo receiver.

Unusual is true DC-coupling. The GTP-500 II amplifies and controls audio flat down to DC, unless you switch-in the Tone Controls or use the AC ("Normal") preamp outputs.

The ADCOM GTP-500 II has a high-output, zero output-source-impedance (less than 0.2 Ω!) dedicated headphone amplifier with a professional ¼" socket.

This is a 2011 test of an unmodified unit built 20 years ago.

Sound

Its sound is neutral, however colored by some channel imbalance from the relatively inexpensive ganged potentiometer used to control volume. More serious preamps like the Apt Holman Preamp and the Quad 34 use genuine laser-trimmed stepped attenuators instead of pots. The ganged pot in preamps like the ADCOM GTP-500 II wholesale for about 30¢, while stepped attenuators wholesale for about $30. This is the biggest audible limitation of most analog preamps.

The balance control is a typical attenuate-one-channel-at-a-time type.

With low-impedance, high-sensitivity headphones like the Ultrasone Edition 8 there is some slightly audible noise from the headphone amplifier.

The Contour (loudness) control is excellent: it's not boomy and doesn't boost the treble. The Contour control is at least as good as those on 1950s and 1960s integrated tube amplifiers that boosted the deep bass and left the rest alone. The GTP-500 II's contour control is uncanny in bringing out astonishing levels of very deep bass when a full-range system is played at very low levels. Likewise, the Bass control also works great, never making the bass boomy. The treble control affects only the higher treble. The tone circuit doesn't degrade the sound — and is bypassed unless deliberately engaged anyway.

All of the Bass, Treble and Balance controls are center click-stopped.

The input selectors are reasonably click-free as inputs are selected. Since the remote makes it easy to select between alternate inputs with dedicated buttons, it's easy to compare different input sources, like your Krell SACD player at the CD input and the CD on the same hybrid disc ripped into iTunes and played remotely from the iPod Touch in your hands via AirPlay with an Apple AirPort Express connected to the TAPE 1 input.

I do lab reports first to make sure that this 20-year-old piece of used gear gotten from a stranger over eBay isn't going to go unstable and blow my ears or tweeters, so you'll see an extensive lab report below. I'll be adding more about how it actually sounds as I spend more time with it.

As I do spend more time with it, it sounds smooth, with a perfect combination of warmth and sweetness, especially when listening at low levels with the Contour engaged. At all levels except those loud enough to duplicate a live performance, the Contour control works fantastically to replace the very deep bass that otherwise goes unheard.

Mechanical

The case is sheet metal, the front panel is anodized aluminum and the knobs and buttons are plastic. The four feet are hard plastic. All markings are painted.

Electrical

It's got a fat 16 AWG power cord, with plenty of extra insulation to impress the innocent, but it's only 5½ feet long.

It has two switched and an unswitched polarized outlet.

It draws about 16 watts as I measured it, and draws 5 watts even when turned-off to keep the remote control receiver alive.

Ergonomics

The real volume control knob works great, and so does the motor controlling it remotely.

The click-stops on Bass, Treble and Balance are wonderful, as are their knobs. It's sad how many other preamps make this difficult with either screwy knobs, or skipping the zero detents.

The black push buttons don't make their positions obvious when viewed any further away than a foot.

Input selectors are debounced: you have to tap them for more than a millisecond or they ignore you. I'd prefer direct-entry buttons to having to run through all the options each time, but tough.

The remote control controls most functions, except for Balance, Bass and Treble, which are analog controls without motors. The remote also lacks the MONO switch, which is a real analog switch on the GTP-500 II, which would have been handy for speaker and head positioning.

The remote only controls the Listening selection. It cannot control the Recording selection, so forget trying to use a tape look as an external-processor loop and then being able to select Listening inputs remotely. (When a tape loop is used as an EPL, the Listen input is set to Tape 1 or Tape 2, and the Listening inputs are selected with the Recording control.)

The motorized remote-controlled Volume pot runs at just the right speed via the remote control. I wished its LED blinked as it moved as it does on my Sony CDP-X303ES, but tough, and that the Volume LED were visible from all angles and that the faceplate indices were illuminated, but tough.

Tuners

The tuners are surprisingly great for real-world use.

Most people never use more than a crappy FM dipole antenna, and with that, the ADCOM GTP-500 II is much freer from distortion and noise than more advanced tuners like the Kenwood KT-917 to which I compared it. Even with a dipole, this ADCOM brings in more cleaner than any of the other tuners I've tried. I haven't tried it with a proper roof-mounted directional FM antenna on a rotor.

The AM tuner, with the included loop antenna, was also quite a surprise. While its frequency response is as crappy as almost every other AM radio, it was far more free from birdies, whistles, static and interference than all of my other AM tuners.

The tuner uses a non-multiplexed vacuum fluorescent display.

It's set back into the case, so the top becomes hidden when viewed from above about a 40° vertical viewing angle.

It's easy to tune, with your easy selection of either Scan or Manual modes for the up/down buttons.

Scan works great, stopping right where I want it to — but it only scans on FM.

The tuner's bright, real-time, 5-red-LED S meter also works much better than the delayed and coarse indications of most other tuners. The 5 segments are well calibrated to show meaningful differences between stations, both AM and FM.

The tuner combines the UNMUTE and HI-BLEND button. It has no automatic hi-blend on weak stations, so if you want to hear weak stations, the strong stations will be hi-blended, or the weak stations will be muted. This isn't as big a deal as it seems, because the hi0blend is much milder than other tuners and this tuner has much less noise when used with crappy antennas in real-world reception than other tuners.

The tuner stores its memory settings with an internal capacitor. It stores its settings for at least 11 days when left unplugged.

I used a state-of-the-art Rohde &Schwarz UPL for these measurements. I tested a 20-year old unit today in 2011.

Unless mentioned otherwise, they are measured with the GTP-500 II set to unity gain (about noon to 1 o'clock on the volume control), measured from the CD input to the LAB output, no filters and no tone controls, loaded with the 200k Ω load of the Rohde &Schwarz UPL.

The traces are color coded for the Left Channeland for the Right Channel. When they don't lie on top of each other, it's due to channel imbalance.

Tracking (left-right channel balance versus volume control setting) is pretty decent for a 30¢ ganged pot, but still has some audible imbalance at lower settings. It's nowhere near as good as the Apt Preamp or the Quad 34.

In this graph below, the scale has been compressed to fit the GTP-500 II's much larger imbalance compared to the greatly expanded scale of the graphs for the Apt Preamp and Quad 34. For those better preamps, I've enlarged their scales fifteen times more than below to show inaudible shifts; in the graph below, you'll probably hear anything more than about 2 dB as the image moving to the right.

No big deal, I set the GTP-500 II's Balance control to a little less than about 11 o'clock to compensate at volume control settings of less than 12 o'clock.

Measured at the LAB output, as driven from the muted output of the Rohde &Schwarz UPL, with its 5Ω unbalanced source impedance feeding the GTP-500 II's CD input:

Volume Control

Noise, A-Weighted

Noise, unweighted

Maximum

-94.7 dBV

-91.7 dBV

Unity (12:20 o'clock)

-96.1 dBV

-93.2 dBV

Minimum

-97.5 dBV

-94.6 dBV

To convert these dBV readings into signal-to-noise ratios, add the voltage of your output signal. For instance, a CD player's maximum output is +6 dBV, so with -94 dBV of noise, there is a 100 dB signal-to-noise ratio.

The volume control is an ordinary design, so there is no noise advantage at lower settings as we see in the
Apt Preamp and Quad 34.
I'd like to see less noise coming out of this ADCOM preamp.

Here is the response of the Contour control at various Volume Control settings:

Contour response, volume control at maximum.

Contour response, volume control at 3 o'clock.

Contour response, volume control at 2 o'clock.

Contour response, volume control at 1 o'clock.

Contour response, volume control at 12 o'clock.

Contour response, volume control at 11 o'clock.

Contour response, volume control at 10 o'clock.

Contour response, volume control at 9 o'clock.

Contour response, volume control at 8 o'clock.

Infrasonic Contour response, volume control at 9 o'clock.

The Contour control boosts all the way down to DC. Watch out!

All these graphs tell us is that the Contour control gives a big boost to the deepest bass, pretty much with the same strength regardless of the position of the Volume control in the softer half of its rotation.

There was no measured change in noise levels, THD or maximum output with the tone circuit in or out.

Tone Circuit Gain

The tone control circuits have very slightly less gain than in the usual deactivated position. This slight change is inaudible, measured at 1 kHz, and is the gain relative to the gain in the usual position:

Left

Right

-0.0909 dB

-0.0520 dB

Frequency Response

The tone control circuit adds one pole (series capacitor), -3 dB at 2 Hz, so we lose DC response with the tone controls, which usually is a good thing.

The 110kHz high-frequency response is unchanged with the tone controls active.

With the CD input selected for the Tape output and fed from the muted 5 Ω source of the Rohde &Schwarz UPL, noise measured -113 dBV A-weighted, and -110.2 dBV unweighted. This is very good; I wish the main outputs were this clean.

THD, CD in to Tape Output at 1 kHz, ADCOM GTP-500 II.

THD, CD in to Tape Output at 1V RMS, ADCOM GTP-500 II.

THD of the tape output is even cleaner than the main outputs, which makes perfect sense since they precede most of the preamp's active circuitry.

Use as an EPL

If we use the Tape Outputs as an external processor loop (select CD to record, feed Tape 1 Out to Tape 1 Input, and listen to Tape 1):

Frequency response is flat from DC to 20,000 Hz ± 0.075dB.

Gain change is -0.2899 dB left, -0.2906 dB right, which is the loss in the tape output.

Maximum output into 600 Ω measures 7.3 VRMS at 0.1% THD at 1 kHz, unchanged with or without the Tone Controls active.

With a 600 Ω load, frequency response is unchanged.

THD into 600 Ω load, ADCOM GTP-500 II.

THD, 1 V RMS output into 600 Ω load, ADCOM GTP-500 II.

Distortion is a little bit more when driving a 600 Ω load, but nothing audible. I'd have no qualms using this preamp to drive 600 Ω loads from its LAB output. The Normal output's AC coupling calculates to -3dB at 56 Hz (I didn't bother to measure), so use the LAB output with 600 Ω loads.

The headphone amplifier is excellent, except for some minor noise that can be audible with high-sensitivity, low impedance headphones best suited to iPods like the Ultrasone Edition 8. With 600 Ω headphones like the Beyer DT 880, this amplifier sings.

For reference, the sensitivity of the 600 Ω (rated) DT 990 is 100dB at 1 VRMS, and 113 dB at 1 V RMS for the 30 Ω rated Edition 8, suggesting 119 dB SPL sinewave maximum for either — at 0.1% amplifier THD.

Headphone Amp Frequency Response

No surprises here, it's as flat as the main outputs, with its own AC coupling: -3 dB at 0.6 Hz.

At 110 kHz, it's response is down only 1.65 dB, or only 0.5 dB less at 110 kHz than the main outputs. I'll skip you the graphs.

Headphone Amp Source Impedance: 0.15 Ω at 1 kHz!

I measured the headphone output source impedance at 0.15 Ω at 1 kHz. This was difficult to do, since I had to null out even the cables involved.

Laymen may be confused by what seem to be 49.9 Ω buildout resistors in the headphone amplifier circuit, however since the negative feedback is taken from after the resistors, the headphone amplifier effectively creates a near-zero output impedance, which is confirmed by my real-world frequency response measurements.

There's a very little bit of background hiss audible (only if music isn't playing) with the highly sensitive 30Ω Ultrasone Edition 8, but with 600Ω Beyer DT880, there is no noise.

Here's where my BSEE and federally-licenced broadcast-engineering magic powers come in. Do you know why we use dBs? Because it makes this all very easy to calculate for useful results. The Edition 8's sensitivity is rated at 113 dB at 1V, so the noise level as heard in the Edition 8 is 25 dB SPL (113dB@1V - 88dBV = 25 dB SPL), and the noise level as heard in the DT880 is 12 dB SPL (100dB@1V - 88dBV = 12 dB SPL). It's easy to hear 25 dB SPL in a closed headphone like the Edition 8 if no music is playing, while we're not going to hear 12 dB SPL in an open headphone, since the ambient noise even in a quiet place like a library is about 30 dB SPL.

Headphone Amp THD

This of course includes the THD of the preamp, from which the headphone amp is fed internally.

The front power switch is just a tap-tap switch to toggle the internal power relays — it's not the actual power switch as you'd expect with this vintage.

The two switched outlets are rated 500 VA. ADCOM suggests you might want to plug amplifiers rated at more than 200 WPC directly into the wall, which means that the 200 WPC GFA-555 II seems tacitly approved. I tried it, and the GFA-555 II is perfectly happy running from the switched outlet. The GFA-555 II usually draws only 50 watts from the wall; the higher ratings on the back of the amp are maximums encountered only under full-power sinewave tests.

There is a turn-on delay relay. The mute relay normally shorts the outputs to ground, and a a few seconds after power-on, the relay clicks-on and removes itself from the circuit.

There's no formal pilot light. The pilot lights are the two input selectors and the volume control LEDs, as well as the tuner VFL display.

The tuner display is always active, even if the tuner isn't selected. Regardless of what inputs are selected, the tuner is always on and responds to tuning button pushes.

The MONO control acts before the balance control, as it is in most hi-fi gear.

The Contour control can be left on all the time. If the deep bass or rumble get too much, the Bass control's effect is very similar, so leaving Contour engaged and reducing the Bass control's setting will let you partially cancel the boost of the Contour control.

The LAB output is DC-coupled, and not a good idea unless you're really afraid of capacitors. Use the Normal output, which is AC-coupled and flat down to 1.6 Hz anyway.

Select the inputs for recording or listening with repeated taps of their buttons. They are debounced, so they will ignore you if tapped too quickly. I'd prefer direct-selection, but tough; the remote control does allow one-push selection of any input.

The AM/FM selector switch has longer throw than others, but it's still just a push button that toggles other circuitry — it isn't doing any switching. It has a longer debounce time, so it will ignore you if tapped too quickly.

To store a station, press ENTER (the LED lights), and tap the location into which you want to store it. To cancel the ENTER command, swap bands from AM to FM and back.

The Hi-Blend and UNMUTE functions share a button. If you leave it out, you'll get the best stereo separation, but weaker stations will be muted, and if you leave it in, even the strongest stations will have their highs blended a bit. The good news is that the Hi Blend is subtle and the tuner is so good that there isn't much noise anyway, so if you need to hear weaker stations and are running from the remote control which doesn't have this switch, no big deal leaving it in BLEND.

Use the Hi Filter to remove distortion from scratchy records or brittle recordings.

The Lo Filter is a waste of time, removing deep bass for no good reason and not doing much to remove rumble. If you need a real rumble filter, the Holman Preamp has a much better one.

For $150 or so today, the ADCOM GTP-500 II is an incredible steal for stereo music enjoyment, especially if you desire a great headphone amp, a tuner, a remote control, better than average Contour and Tone controls, or a capacitor-free signal path.

Its biggest downfall is its cheap volume control pot that sometimes requires a minor tweak of the balance control at lower levels — but its never that far off. If you've found this report helpful, this free website's biggest source of support is when you use these links, especially this link directly to the ADCOM GTP-500 II at eBay (see How to Win at eBay), when you get anything, regardless of the country in which you live. Thanks! Ken.

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